1. Field of the Invention
This invention relates to electrolytic copper foil, more particularly, to such copper foil produced on a rotating drum cathode machine and having its shiny side (drum side) modified by a copper gilding layer electrodeposited thereon, and still more particularly, to such copper foil wherein a copper bonding treatment is formed both on the matte side and the modified shiny side of the foil.
2. Background of the Invention
Electrolytic copper foil produced on rotating drum machines, prior to being treated, is usually referred to as raw foil. The raw foil is pink in color and has two distinctly looking sides--a "shiny side", the side which was plated onto the drum surface and then stripped is smooth, while the other side, the side which was facing the electrolyte and the anodes, is referred to as the "matte" side since it has a velvety finish, due to the difference in the growth rate of differing crystal faces during electrodeposition of the raw foil. The matte side surface, at this stage has a very fine scale micro-roughness and a very specific micro-topography. Viewed under high magnification of a scanning electron microscope, it is composed of peaks and valleys. The peaks are closely packed cones or pyramids. The cone's height, slant, packing and shape depend, as is well known, upon closely controlled independent variables of foil thickness, current density, solution composition and temperature and the type and concentration of the addition agents and the like.
In the fabrication of copper-clad laminates for printed circuit boards (PCBs), copper foil is bonded to polymeric substrates (composite materials: epoxy, polyamide and other similar resins reinforced with glass fiber fabric) by means of mechanical interlocking at the interface between the two materials. To achieve a high degree of interlocking, the bonding side of the foil is provided with a "bonding treatment".
Various patents directed to bonding treatments for copper foil disclose, for example, that one or both sides of the foil which is to be bonded to the substrate is subjected to the bonding treatment (U.S. Pat. No. 5,207,889), or that such treatment for copper foil to be used for lamination to a substrate comprises electrodepositing a dendritic layer of copper followed by a gilding, or encapsulating, layer of copper on the side of the foil that is to be laminated to the board (U.S. Pat. Nos. 3,857,681, Re. 30,180 and 4,572,768).
Such bonding treatment is an extremely dense coating of spherical micro-projections, which is usually electrodeposited to the matte side of base copper foil. This matte side, in itself, is composed of densely packed, micro-conical or micro-pyramidal shapes, which form a micro-topography of peaks and valleys. The peel strength of cooper foil (force necessary at separate, or pull away, the foil from polymer substrate) will depend on the shape of individual micro-projections, their mechanical strength and hardness, density per surface area, and their distribution over micro-peaks and micro-valleys of the matte side of the base foil. In turn, all the factors listed above will depend on the conditions under which the treatment is electrodeposited and the micro-topography of the base foil. Usually, processes for the electrodeposition of bonding treatments employ plating conditions which give rise to high cathodic concentration polarization, namely high current density, low copper concentration and temperature of the electrolyte. Often, the addition of other ionic species, such as chlorides or arsenic, are used to enhance tree-like (on a microscopic scale) structures, which are good for bonding. As such deposits are, by nature, good for increasing surface area, but weak mechanically, the second step of the treatment, locking or encapsulation, usually follows the first step, in order to improve the treatment's mechanical resilience. The treatment operation is conducted in the machines called treaters. Rolls of raw foil are placed at an unwinding station of the treater and fed into the treater by means of driven rollers (similar to the way in which a web of paper is handled in a printing machine), rendered cathodic by means of contact rollers, and passed in a serpentine fashion through a series of plating tanks, facing a rectangular anode in each tank. Each tank has its own supply of appropriate electrolyte and its d.c. power source; between the tanks, the foil is thoroughly rinsed on both sides. The purpose of this operation is to electrodeposit micro-projections of complex shape (visible only under high magnification microscope) on the matte side of the foil, which ensure that the foil may be firmly anchored to the base polymeric materials used in PCB's fabrication. The last tank of the treater provides the foil with a so-called stainproof layer, which prevents staining, tarnishing and oxidation and imparts long shelf life on the foil. Upon leaving the passivation tank, the foil is subjected to the final rinse, dried and coiled into rolls by take-up driven shaft-rollers. At that stage, foil is a finished product. Obviously, to be capable of handling a very thin and fragile copper foil in a manner described above, treater machines are custom built and very carefully engineered. A system of driving the rollers and the foil, web guiding, tension control devices, synchronization at peripheral speed of all guiding rollers, precise machining of "contact rollers" which pass electric current into the foil etc., are the elements of treaters design and engineering which together combine to make treaters very sophisticated and expensive machines.
The basic raw material for manufacture of printed circuits is a laminate clad with copper foil. It is comprised of a thin copper foil firmly bonded to a polymeric, dielectric (insulating) base material. This "bonding" operation is conducted in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid upon sheets of "prepreg" (e.g. glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press having heated pressing plates, while-the two materials are pressed-together under high pressure. At elevated temperatures, the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface. This is followed with a second cycle, when both materials are cooled while the pressure is being maintained. The resin solidifies in the irregularities of the foil surface and both materials are firmly bonded together and are very difficult to pull apart. The "peel strength" between both materials is high, because the bonding side of the copper foil is provided with the bonding treatment. High peel strength is a characteristic of the highest importance since the mechanical support of the circuit elements as well as the current carrying capability of PCB's is provided by the copper foil polymer joint. It is essential that the foil is bonded very tightly and securely to the laminate and also that such an adhesive joint can withstand all the manufacturing steps in PCB's fabrication without a decrease of the initial adhesion, which, moreover should remain constant throughout the service life of the PCB. It is a well-established fact that to achieve satisfactory adhesion in bonding operations, metallic substrates have to be pretreated. A satisfactory pretreatment must not only give the appropriate initial level of adhesion but must also provide "durability". The adhesion must not be seriously affected by environmental conditions, specifically, processing chemicals used in PCB's fabrication, elevated temperatures, high humidity or water. Obviously, copper foil-polymeric substrate bond strength is more and more important as fine-line applications become more common. The adherence between copper foil and the substrate must not only be adequate initially, but ought to be durable and permanent. The usual testing of the peel strength of the sheared 1 cm (or 1 inch) wide specimen of the clad laminate offers only a broad idea of the reliability of the joint to the circuit designer. The permanency of the peel strength under various environmental conditions encountered in PCB's fabrication and their service life is probably no less important. Electrodeposited bonding treatment and its role represent a very special case of industrial application of the electrodeposit. In electrodeposition in general, the adherent, compact deposits are of practical use and the loose, powdery or dendritic deposits are regarded as a nuisance. Powdery electrodeposits, however, have their own technical applications, particularly in the field of powdered metallurgy or catalysis. We have found such deposits to be convenient for providing copper foil with good bonding characteristics.
The parameters of the dendritic (treatment's first step) deposition are purposefully set as to encourage the growth of powdery/dendritic layers. They are:
Low copper concentration PA1 Low temperature PA1 High current density PA1 (a) an electrolytic copper base foil having a matte side and an opposite shiny side; PA1 (b) a first copper bonding treatment on the matte side formed of (i) a first electrodeposited dendritic copper layer on the surface of the matte side, and (ii) a first electrodeposited copper gilding layer on the dendritic layer, forming a treated matte side; and PA1 (c) a micro-roughening copper layer on the shiny side formed of (i) a second electrodeposited copper gilding layer on the surface of the shiny side, (ii) a second electrodeposited dendritic copper layer on the second gilding layer, and (iii) a third electrodeposited gilding layer on the second dendritic layer, forming a treated shiny side.
All these factors lead to high cathodic polarization, or poor mass transport, which enhances the dendritic (tree-like) character of the deposit. What makes the treating process even more complex, is the fact that it is electrodeposited on the matte side of the base foil surface which is not truly flat, but composed of microscopic peaks and valleys. In general, the structure of a deposit depends strongly on the rate of formation of new nuclei and the rate of growth of the already existing crystals. The grain size is especially affected by the ratio of these two rates. If the rate of nucleation is small and the rate of crystal growth large, a deposit with coarse grains results. In the contrary case, the deposit is fine grained. Powder formation can then be regarded as a limiting case in which the rate of nucleation is large and the growth of crystals, particularly their intergrowth, is strongly inhibited. One could add that between the extreme cases of highly dispersed powder and an even, well-adhering deposit, many intermediate cases may occur. Only a narrow range of copper particle size and shape combination is well suited to be the foil bonding. We have mentioned before that powder formation is brought about when the rate of nucleation is large while the growth of crystals is inhibited. These conditions are the consequence of the depletion of copper ions near the cathode (matte side of the foil). It is well established that the depletion of the metallic ions near the cathode is a decisive factor in powder formation. More precisely, powder formation starts when the limiting current is reached or approached, e.g., when the concentration of metallic ions at the cathode-solution interface is zero. The limiting current and, more specifically, the depletion near the cathode, are governed by mass transfer processes.
Bonding treatment may be effected by subjecting the matte side of the "raw" foil to four consecutive electrodeposition steps. The first consists of the deposition of the microdendritic layer which enhances, to a very large degree, the real surface area of the matte side, and thus enhances the foil's bonding ability. This is followed by electrodeposition of an encapsulating (gilding) layer, whose function is to reinforce mechanically the dendritic layer and thus render it immune to the lateral shear forces of liquid resins in the laminating stage of PCB's fabrication. The encapsulating step of the treatment is very important, since it eliminates the foil's tendency toward "treatment transfer" and the resulting "laminate staining" which can cause a decrease of the laminate's dielectric properties. Such a dendrites-encapsulation composite structure should be characterized by high bond strength and the absence of treatment transfer. The treating parameters which assure just that are relatively narrow. If the amount of gilding deposit is too low, the foil will be given to treatment transfer, if on the other hand gilding layer is too thick, a partial loss of peel strength may be expected. These first two steps of the treatment are composed of pure copper, in the form of microscopic, spherical micro-projections. The electrodeposition of this copper bonding treatment is followed by deposition of a very thin layer of zinc or zinc alloy, a so-called barrier layer. Its purpose is to prevent direct copper-epoxy resin contact, and that is why the zinc-alloy layer (which during lamination is converted to alpha brass) is called the barrier layer. If the bonding treatment composed of copper only is subjected to lamination with epoxy resin systems, it tends to react with amino groups of the resin at the high laminating temperatures. It, in turn, creates moisture at the foil-resin interface, causing a harmful effect of "measling" and possibly delamination. A barrier layer which is plated over all-copper treatment prevents these harmful effects entirely. All three stages of the treatment mentioned above, effected by means of electrodeposition, change the geometry and morphology of the matte side of the foil, assuring the mechanical strength of the surface region, as well.
The electrodeposition of the treatment is typically followed by an electrochemical stainproofing which changes the surface chemistry. As a result of this step, the bonding surface is rendered chemically stable. This operation removes weak surface films, which can greatly decrease the adhesion of the solids, and replace them with a stable film of controlled thickness, responsible for imparting "durability" of its properties on the treated surface. The film serves as an undercoat for subsequent bonding. The same stainproofing step protects the shiny side of the foil against atmospheric oxidation.
Contemporary bonding treatments were invented in the early seventies and the major foil manufacturers are using the same techniques today. The changes that have occurred in the intervening years pertain, by and large, to the composition of the barrier layers, and have been made to accommodate technical needs imposed by the emergence of new polymeric dielectric substrates used in the manufacture of PCB's. For example, polyimide substrates introduced to the printed circuit industry fairly recently require a much higher laminating temperature than the epoxy pre-pregs. Consequently, foil manufacturers modified this portion of the overall treating processes in order to achieve the desired composition and performance of barrier layers for the foils that are destined for polyimide applications. Barrier layers on polyimide-grade treatments have to withstand much higher laminating and post-bake temperatures, compared to the treatments destined for epoxy applications. High temperature at the metal-polymer interface can subject the metal surface to oxidation with the attendant partial loss of adhesion. A well designed barrier layer will be self-protected along with the underlaying all-copper treatment from heat oxidation and the loss of bond. Other changes in the technology of the bonding treatment are occurring as well. For example, some major foil manufacturers build their new treaters with a larger number of individual plating tanks, in order to apply twice the sequence of dendritic deposit followed by encapsulating deposit. Thus, quite often, the first four tanks of the treater are devoted to the application of micro-roughening treatment that consists of a dendritic layer followed by an encapsulation layer, and this composite plural layer is repeated twice. This practice is aimed at being able to run the treater at greater speeds, since the initial capital outlay for the construction of the treaters is very high today. Conversely, the larger the number of tanks, with the treater run at more traditional speeds, permits deposition of a greater mass or weight of the treatment to assure acceptable peel-strength on so-called "difficult to bond to" polymeric substrates that aim at higher glass transition temperatures. These substrates, which are often a blend that involves multifunctional epoxies, BT resin, polyimide etc. usually require an increased amount of bonding treatment to assure adequate peel-strength. It should be remembered that aside from its bond-enhancing microstructure, the amount of treatment per surface area of copper foil is also an important factor.
With the advent of miniaturization, very densely packed printed circuit boards are needed. Miniaturization often requires that the copper foil conductor, or track lines, of today's printed circuit board be as narrow as 5 mils, or less. The degree of high definition of fine line circuitry depends on the quality of copper foil manufactured for the electronic industry, particularly on surface quality of both sides of the foil. One side of the foil, whose role is to firmly anchor track lines to the polymeric substrate, is provided with a bonding treatment. After the foil is bonded to the substrate the other side of the foil, which forms the outer surface of copper-clad laminate, is used for image patterning.
It is the practice in manufacturing printed circuit boards from copper-clad laminates to form an image of the desired printed circuit pattern on the exposed copper surface of a laminate by a photographic technique which leaves the desired pattern formed of a photo-resist material on the surface of the copper.
It will be appreciated that for the photographic imaging to be sharp and precise, the photo-resist has to spread well on the foil's surface and adhere well to this surface.
It is a practice in manufacturing PCBs to roughen the exposed surface of the copper to achieve good resist adhesion. This roughening also removes tenacious stainproof films which foil manufacturers apply to the foil to protect it from oxidation and staining before it reaches the user. Photo-resist does not adhere to the stainproof films, which therefore have to be removed. Thus, roughening of the foil surface serves the purpose of removal of stainproof film, as well as changing the copper surface topography from smooth to micro-rough, to facilitate photo-resist adhesion which is a condition of good definition of track lines.
This roughening is performed by either mechanical means, e.g., abrasion by brushes, scrubbing with pumice, or chemical means (so-called micro-etching), which is accomplished by subjecting the copper surface of copper-clad laminates to the etching action of oxidizing mineral acids. Such acids attack the smooth surface of the foil along the copper grain boundaries, thus creating pits and pores and changing the copper surface from smooth to micro-rough.
Multi-layer printed circuit boards, so-called MLBs, now dominate the printed circuit board market, since they permit achieving the highest functional density in electronic packaging. In the fabrication of MLBs, copper foil is laminated (bonded) to polymeric substrates twice. First, thin, double-sided copper clad laminates are produced. These laminates are then subjected to image patterning and etching away of unwanted copper to produce the desired patterns of circuitry. Several layers of double-sided boards prepared in such a manner are stacked together, with sheets of prepreg (glass reinforced polymeric resin composites) inserted in between to dielectrically separate adjacent boards from one another. Such a stack of circuit boards and pre-preg is then laminated together, in the so-called "B-stage lamination", to form a monolithic multi-layer board. Later, holes are punched or drilled through the board in pre-arranged places and so-called thru-hole plating of copper is used to ensure the electrical interconnection between all layers of copper-track conductor lines.
It is a practice in the fabrication of MLBs to subject the inner layer boards, with their patterns of circuitry, to a so-called brown-oxide treatment, which changes the micro-topography of the top surfaces of the track lines to improve their bondability to the polymeric pre-preg. This brown oxide treatment is typically produced by immersing the boards in an alkaline solution of sodium chlorite, which by its oxidizing action causes the conversion of metallic copper on top surfaces of exposed copper tracks into cupric oxide CuO with a possible admixture of cuprous oxide Cu.sub.2 O, depending on the type of the bath and operation conditions.
This oxide coating grows in the form of dendritic crystals, perpendicular to the surface of the copper tracks. Thus, the surface area available for bonding to polymeric substrates is increased and improved "bondability" is achieved.
We have found that adhesion between the copper surface and the photo-resist is a crucially important component in the subsequent processing of boards having fine line circuitry, because the edge of the photo resist defines the area of copper whereon all subsequent processing will be carried out. This processing can be either the removal of copper by etching or the addition of copper required in so-called through hole plating. If adhesion between the resist and the copper is inadequate then the boundary between the original copper and the etched or plated copper will become irregular so that there is the possibility that variations in the cross sectional area of the circuit path could occur. It therefore becomes clear that any adhesion-promoting roughening of the shiny side of the copper must be carried out by techniques that are controllable within close parameters for optimum utility. This is particularly important as line widths of the circuitry decrease.
Accordingly, we have conducted considerable experimentation with ways of improving such adhesion. As a result of this work, we have developed a novel electrolytic copper foil having its shiny side modified by a micro-roughening deposit which comprises an electrodeposited copper gilding layer, which deposit enables superior wetting by and adhesion to photoresist materials and is unusually well-suited for use in producing copper-clad laminates to be used in the manufacture of circuit boards having fine line circuitry. This modified shiny side foil is sometimes hereinafter referred to as "MSS foil".
An object of the present invention is an electrolytic copper MSS foil having a modified shiny side providing superior adhesion to photoresist materials. Another object of the invention is a copper-clad laminate wherein the matte side of such MSS foil is laminated to a polymeric substrate through a copper bonding treatment electrodeposited on the matte side of the foil. A further object of the invention is an electrical circuit formed on the copper-clad laminate by applying a photoresist material on the modified shiny side of the foil, image patterning and etching to form the circuit.